It is known to provide containers, into which a cover can be recessed at the top thereof, of spherulitic cast iron which can have surfaces with open pores and which are formed by casting, to accommodate nuclear materials for storage and transport to disposal sites, or to hold irradiated fuel elements until they can be processed.
It is also known that the porosity of the cast iron can pose a problem and hence it has been proposed to coat the cast iron with a metal in an effort to seal the pores thereof. The sealing layer can be applied to the seat receiving the cover or to the cover as well.
In the past, radiation shielding transport containers of this type have been used to accommodate irradiated fuel elements by immersion of the container in the fuel element basin of the nuclear reactor which generally is filled with water and the fuel elements are introduced into the container under water. The basin has usually a cladding of stainless steel, for example 18/8 chromium nickel steel.
For electrochemical reasons, upon introduction of the container of cast iron into such a basin, the container will form a galvanic element and especially ferritic iron will be lost from the cast matrix into the solution. As a consequence, the stainless steel cladding of the fuel element basin will be corroded and the surface of the cast iron container will be detrimentally affected.
To avoid this problem, it has been proposed to provide a sealing layer on the surfaces of the container which will come into contact with the water. This hinders the formation of the container as a galvanic element and solubilization of ferritic iron from the cast iron structure and also, therefore, reduces the corrosive effects. By and large in the past nickel and nickel alloys have served as coatings for the cast iron for this purpose.
The coating has been applied to the cast iron structure by galvanic techniques, i.e. electroplating. For this purpose, galvanotechnical apparatus must be used and because of the large size of the containers employed, galvanotechnical apparatus for coating the containers are highly expensive.
From a practical point of view, it has been found that electroplating techniques can be used effectively only for very thin layers so that layers of 200 micrometers or greater in thickness cannot readily be grown on such cast iron surfaces.
Because of unavoidable mechanical, thermal or corrosive effects, it has been found that claddings applied galvanotechnically to the cast iron structures have more or less point-form open locations or defects.
Investigations which have not become part of the published literature have shown that these open locations tend to form above open pores which are present in the surface of the cast body. Apparently these defects in the coating are unavoidable because the open pores, by reason of the electrical potential at and around the pores during electroplating, cause gaps in the electrodeposited coating at least initially so that the pores are not filled with the nickel or nickel-based alloy, but rather appear to be bridged by relatively thin and mechanically sensitive layers.
The areas at which the pores are located are those bridged by a coating which does not penetrate into the pores and is mechanically sensitive at these locations so that even minimal stresses can break away the coating, even if the coating is extremely thick, for example of a thickness of 1.5 mm, 2.0 mm or more, to expose the pores and create incipient defects in the coating.
Without such thick coatings, moreover, bridging of the pores cannot be ensured so that earlier coating techniques are not very reliable and are very expensive.